The invention relates generally to high power machines and more particularly to slip ring brush systems.
In high power, variable speed machines such as described by commonly assigned Runkle et al., U.S. Pat. No. 5,742,515, rotors are excited through electrically conductive slip ring assemblies. Conventional slip ring and brush assemblies transfer power at low voltage, moderate current levels and relatively high speeds. It would be useful to have assemblies that can accommodate thousands of amperes even when rotating at low speeds (about zero (0) to about one hundred (100) rotations per minute). In conventional slip ring and brush assemblies, rotating at low speeds may lead to localized overheating, marking, and deformation.
U.S. Pat. No. 1,157,885 of the London Patent Office to Brown, Boveri & Co. Ltd. describes a technique for “causing the brushes to be moved back and forth” by “using a device which serves to produce a periodic rotary motion of those parts of the machine on which the brushes are mounted.” To accomplish this motion, “brush bridges are connected with the output shaft of a differential gear that changes their position, and an input shaft to this gear is equipped with means for altering the central position of the brush bridge, a second input shaft to this gear being provided with means for effecting a periodic back and forth rotation with variable amplitude.” Azimuthal motion is thus achieved. The resulting assembly of this technique is difficult to control and to run at high voltages.
“Rotary Converters,” Westinghouse Electric & Manufacturing Co., Circular No. 1028, April 1903, describes an embodiment wherein axial motion is achieved between brushes and a commutator on a rotor shaft by moving the entire rotor axially in an oscillatory manner. Moving the entire rotor shaft is awkward, may be harmful, requires significant power, and can limit the amplitude of motion.
Other limitations of variable high speed machines relate to the non-uniform current distribution through the brushes that contact the slip rings as well as the rotor winding leads that couple rotor windings to slip rings. Because the brushes typically are coupled by a common, single electrically conductive lead, the current varies between brushes depending upon the proximity to the conductive lead. Rotor windings are also coupled together prior to attachment to the slip rings and similar current variations result. Although stators are sometimes connected to stationary connection rings with separate leads, as shown in Lawrence, Principles of Alternating-Current Machinery, 4th Ed., McGraw-Hill Book Co. 1953, such configurations are not used for slip rings of wound rotor machines.